1. Field of the Invention
The present invention is directed generally to systems for monitoring the operating parameters of a steam turbine generator and, more particularly, to systems for monitoring the clearance between the fixed and rotating parts within the turbine.
2. Cross Reference to Related Applications
The present application is related to U.S. Application Serial No. 277,832 entitled Turbine Blade shroud Temperature Monitor filed and assigned to the same assignee as the present invention. (WE Case no. 54,472)
3. Description of the Prior Art
Most turbine blade rows are shrouded to provide some measure of sealing between the stationary and rotating portions of the turbine. Shroud segments are arc-shaped structures connected in series to form a circular shroud around the tips of the blades in a blade row. Each shroud segment is typically riveted to three or more blade tips. Thus, a number of shroud segments are needed to completely encircle a row of blades.
The clearance between the stationary seal of the turbine and the shroud segments must not be so great as to allow an excessive amount of steam to pass between them and thereby reduce the efficiency of the turbine. On the other hand, clearances cannot be too small because highcentripetal loading and high temperatures may cause shroud segments to lift or grow radially. Such shroud lifting or radial growth can cause shroud segments to rub the stationary seal and may eventually cause seal, shroud, and blade damage. Thus, it is desirable to monitor the clearance between the shroud segments and the stationary seal to prevent damage to the turbine.
A system for monitoring such clearances is disclosed in U.S. Patent Application Serial No. 199,633 entitled Turbine Blade Shroud Clearance Monitor filed 27 May 1988 and assigned to the same assignee as the present invention. (WE case no. 54,160) The invention disclosed in that application uses a non-contact type of sensor to produce a signal representative of the distance between each shroud segment and the stationary portion of the turbine. The signals thus produced may be processed in a number of different ways, e.g. comparing the average clearance value of each shroud segment to the average clearance values of the two adjacent shroud segments, comparing the average clearance value of each shroud segment to the average row clearance value, comparing the average clearance value of each shroud segment to a predetermined critical value, etc. However, the system may suffer from unknown effects caused by varying temperatures, varying rotor speeds, and the use of different types of metals in the construction of the rotating portion of the turbine. Thus, the need exists for a shroud clearance monitoring system which is immune to such unknown effects.